Optical second-harmonic generation and ellipsometry were used to study molecular alignment in an interfacial region and the deviation of the layer normal from the surface-rubbing axis in the smectic-/4 phase of a ferroelectric liquid crystal. The results can be well explained by the surface electroclinic effect. PACS numbers: 6l.30.Gd, 42.65.Ky, 64.70.Md, 68.45.-v The problems of interfacial properties of liquid crystals (LCs) are not only of fundamental interest but also of practical importance to the design and construction of LC devices. It is well known that an LC film can be oriented by a rubbed polymer surface [1]. It has been recently shown that the rubbed polymer surface effectively aligns the first LC monolayer on the surface, and then via LC molecule-molecule interaction, the average molecular orientation (the director) in the bulk is aligned [2,3]. Recently, however, it has been found that in the smectic-/^ iSmA) phase of chiral molecules the bulk LC director and hence the smectic layer normal deviate from the rubbing axis by an angle 0, which can be as large as 18° [4,5]. Nakagawa et ai [4] observed that the sign of 0 correlates with the sign of spontaneous polarization in the smectic-C* (SmC*) phase. Based on this observation, they proposed that the deviation originated from the electroclinic (EC) effect [6] in the SxnA phase induced by a localized surface field. Using a different LC material, Patel, Lee, and Goodby [5]found a large 0 and suggested that it is too large to be described by the surface EC effect. They proposed an alternative mechanism based on molecular twist via chirality and reduction of smectic ordering at the surface. Both models qualitatively explain some aspects of the experimental observations but not others so that a complete understanding of the phenomenon is still lacking.
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The transient ionic current in a ferroelectric liquid crystal (FLC) cell was measured as a function of applied voltage at different temperatures. The measured current shows a delay peak which decays rapidly to a steady-state value. The steady-state current exhibits a thermal activated behavior with an activation energy of 0.94 eV, and depends exponentially on the square root of the field strength similar to the Schottky effect. To describe the transport behaviors of the ions, a model with ionization-recombination equations which include a field-dependent ionization coefficient, and a continuity equation for the accumulated surface charges has been introduced. According to the model, the current decay time and the peak position of the delay peak current can be used to determine the ion mobility. Two ionized species have been identified in the FLC medium: a weakly ionized species with a relatively constant mobility of 2.0×10-7 cm2/V·s, and a highly ionized species with a room temperature mobility of 4.1×10-7 cm2/V·s and a thermal activation energy of about 0.71 eV. Detailed calculation of the ionic current and comparison with experimental results are given.
The crystallographic and magnetic properties of Tm6Fe23 before and after hydrogen absorption were studied by means of x-ray diffraction, bulk magnetic measurements and '69Tm and "Fe Mossbauer spectroscopy. The data were analysed in terms of a mean-field model. Coupling constants describing the magnetic interaction between the Fe moments and between the Tm and Fe moments were derived for Tm6Fel3 as well as for its hydride.
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